Semiconductor devices, such as, integrated circuits, chips, ball grid arrays, multi-chip modules and microelectronic packages are connected to one another via printed circuit boards. The term printed circuit board is used here in a generic sense and include all types of boards that hold chips and other electronic components. Typically, a printed circuit board is made of reinforced fiberglass or plastic and interconnects components via copper pathways. The main printed circuit board in a system is called a system board or motherboard, while smaller ones that plug into the slots in the main board are called boards or cards.
A semiconductor device generally has a planar surface with several contacts or leads arranged in a pattern. A printed circuit board generally has solder wettable contact pads having some solder deposited thereon and arranged in patterns that correspond to the pattern of contacts on the semiconductor devices to be mounted on the board. Typically, a semiconductor device is mounted on a printed circuit board by placing the device contact points on corresponding board contact pads and then subjecting the semiconductor device and printed circuit board combination to a thermal cycling process. The thermal cycling process first heats the solder to its liquidus temperature thereby causing the solder to flow, and then cools the solder to its solidus temperature thereby causing it to solidify so that a solder interconnection joint is formed and the device is attached to the board.
Generally, the semiconductor device and the printed circuit board are made of different materials. For example, the semiconductor device may be a plastic resin encapsulated ball grid array or a ceramic encapsulated ball grid array, whereas the printed circuit board may be made of an epoxy resin. These materials have significantly different coefficients of thermal expansions, which means that the materials expand and contract differently when heated or cooled over the same temperature range. Accordingly, using prior art interconnection methods as described above, when a printed circuit board and a semiconductor device combination is subject to thermal cycling for solder reflow, the board and the device expand and contract at different rates. This thermal mismatch caused by the difference in coefficient of thermal expansion generates a substantial amount of mechanical stress in the device, the board and the solder interconnection formed upon completion of the thermal cycling process. The mechanical stresses introduced into the solder interconnection causes fatigue within the solder interconnection resulting often in failure of the interconnection. Additionally, the thermal mismatch caused by the difference in coefficient of thermal expansion of the materials may cause the substrate to warp.
Also, during normal operation of a semiconductor device, the flow of current through the solder joints of the interconnection causes heat to be generated. The resulting heat causes the board and the device to expand and contract at different rates due to the difference in their respective coefficients of thermal expansion. This thermal mismatch caused by the difference in coefficient of thermal expansion generates a substantial amount of mechanical stress in the solder interconnection. The mechanical stresses introduced into the solder interconnection causes fatigue within the solder interconnection resulting often in failure of the interconnection.
Additionally, semiconductor devices, such as ball grid arrays have been getting larger and heavier as more and more functionality and circuitry are built into them. During the thermal cycling process to reflow the solder, the weight of the semiconductor device often causes the solder interconnect to collapse or semi-collapse and distort. Collapsed solder interconnects pose several processing problems and also often lead to shorting of adjacent pads of the printed circuit board. Additionally, a collapsed or distorted solder interconnection joint has built in mechanical stresses that produce fatigue in the solder joint and may result in failure of the solder joint.
Prior art methods have attempted to solve the problem of solder joint collapse and a high wattage solder joint requirement by using solder alloys that are harder and are hence better able to support the weight the of a heavy semiconductor device. However, the higher liquidus temperature of such alloys cause thermal degradation of the materials of the semiconductor device and the printed circuit board which are typically unable to withstand the higher temperatures.
Accordingly, there is a need for a semiconductor device and printed circuit board interconnection that does not collapse under the weight of the semiconductor device and is not subject to built in mechanical stresses caused by coefficient of thermal expansion mismatch of the materials from which the semiconductor device and the board is formed.